Ad networks and Web sites constantly track your behavior as you surf the Web, recording what sites you visit, what pages you visit on sites, and what kind of content you like to view. If you'd like to keep your personal Web preferences to yourself, get the free Firefox addon PrivacyChoice Opt-Out, which lets you stop more than 100 companies from tracking your behavior.
Install the add-in, then choose Tools-->PrivacyChoice Opt-Outs in Firefox, and you'll be sent to a Web page that lists more than 100 companies and Web sites that track your behavior, and from which you can opt out. You can find information about each Web site, such as whether it shares information about you with partners, whether you are anonymous when you visit the site, and so on. On the Web page you can then opt out of all of the companies, or only the ones you select.
Privacychoice.org also makes another Firefox add-in, TrackerWatcher, which lets you get similar privacy information about sites you visit as you surf the Web. Using PrivacyChoice Opt-Out in concert with TrackerWatcher makes an excellent one-two punch for protecting your privacy.